The petroleum crisis in the early 1970's was the impetus for significant innovation in wave energy conversion systems. A lack of practical solutions or reasonable prospects of efficient and robust technologies, plus declining oil prices, eventually led to a general disenchantment in the viability of wave-energy conversion.
Research continued at a few largely academic centers and over the past twenty-five years a great deal has been learned. Both theoretical understanding of sea waves and technical expertise in related marine engineering has gained immeasurably from the offshore oil and gas industries during the same period. Growing concern with global climate change has led to an increased sense of urgency in the quest for commercially viable renewable energy sources.
The theoretical potential of wave energy has been recognized for many years. The size of this resource has been estimated to be 219 gigawatts along the coats of the European Union, or more than 180 terawatt hours each year. The wave power off the west coasts of Ireland and Scotland, where the winter resource is approximately twice that available during summer months, ranks with the highest levels per kilometer in the World.
Wave energy is lost by friction with the sea bottom as the sea becomes shallow, with water depths of half a wavelength or less. This is most pronounced where wavelengths tend to be long, as off the NW coast of Europe.
Research and development into wave energy converters (WECs) over the past twenty-five years, plus the knowledge and practical experience gained from the off-shore oil and gas industries, has now reached a stage where robust and effective wave energy converters with installed capacities of one megawatt and greater are being developed.
The wave energy resource may be split into three broad categories, based on where the energy from waves may be recovered: 1. in the open sea, i.e. offshore; 2. on or close to the shore line, i.e. on-shore or inshore; 3. outside the normal area of breaking waves but not in the deep ocean, i.e. near shore.
A fourth category, not generally considered in the context of wave energy converters, but which may be of relevance to this present invention, is waves or surges in a liquid contained in vessels and tanks.
The very large number of devices and concepts proposed to date has been classified and described in summary form for the Engineering Committee on Oceanic Resources by the Working Group on Wave Energy Conversion (ECOR draft report, April 1998). This follows a similar classification based on the intended location, i.e. off-shore, near shore to off-shore, and on-shore.
Wave Energy Converters (WECs) may also be classified in different ways according to their operating principle and the ways in which they react with waves. In terms of practical application, only a very few types of device are presently, or in the recent past have been, in use or under test in European waters.
By way of illustration, two different but overlapping classes will be briefly commented on: the Oscillating Water Column (OWC) devices and Point Absorbers, the latter being the relevant class in the present context.
OWC devices are typically those where the wave is confined in a vertical tube or a larger chamber and, as it surges back and forth, drives air through a power conversion device. Megawatt-scale OWC devices are now at an advanced stage of development.
One such device, being built in a rocky gully on the western shore of Pico in the Azores, is a reinforced concrete chamber partly open at one side to the waves, and with two turbines above and behind through which the confined air is forced. These are specially developed Wells turbines (one with variable blade pitch) and on the whole would seem to be the best-developed and perfected conversion system available today. It is, however, unlikely that any one such installation will have an installed capacity greater than two megawatts and the number of suitable sites has to be extremely limited.
Point absorbers may react against the sealed (therefore necessarily sited near-shore), or be floating and self-reacting. Theoretical analysis has greatly increased our understanding of point absorbers.
Point absorbers are usually axi-symmetric about a vertical axis, and by definition their dimensions are small with respect to the wavelength of the predominant wave. The devices usually operate in a vertical mode, often referred to as ‘heave’. As such they are capable of absorbing energy arising from changes in the surface level rather than from forward motion of breaking seas.
The theoretical limit for the energy that can be absorbed by an isolated, heaving, axi-symmetrical device has been shown to depend on the wavelength of the incident waves rather than the cross section of the device, i.e. from the wavelength divided by 2. pi. Thus the wavelength is a critically important criterion, resulting in the attraction of locating the point absorber devices well outside the region of breaking waves, and where they will be open to long wavelength ocean swell or ‘heave’.
A point absorber device may react against the inherent inertia of one of its components, or against the bottom of the sea. Thus, point absorbers may be deployed near-shore in contact with the sea-bed or, in the case of self-reacting devices, near-shore or off-shore.
Small-scale practical point absorbers such as fog horns and navigation buoys, both of which may incorporate OWCs, have been in use for decades. Typically these have a power of a few hundred watts.
One new point absorber device, now claimed to be capable of generating of the order of a megawatt, has been described. This is based on the buoyancy variations of a submerged, partly air filled, rigid vessel open at the bottom. Initially the device is floating with neutral buoyancy at a certain depth.
If a wave passes above it the pressure around this vessel increases and water will flow into the vessel, displacing the air or gas inside (which is free to flow to a large reservoir or to similar devices linked by pipelines), decreasing the air volume in it and hence its buoyancy. The upthrust experienced has decreased in proportion to the volume of water displaced, i.e. Archimedes' principle. The partially filled vessel will start to sink. When a trough passes above it the reverse process occurs, and the vessel tends to rise to recover its rest position.
The size of the forces exerted will depend on the extent of the water surface within the vessel, the amplitude of the wave and the frequency of waves. The wave energy transformer is described in terms of two similar containers, horizontally displaced, such that the gas displaced from one container passes to the second. Essentially the gas, being free to move between two or more similar devices remains under constant pressure, as required by the depth below the surface.
This is a heavily engineered device, one that will not readily flex with the lateral movements of water as found below waves, it is not independent of the seabed and is not independent of tidal changes in mean sea level. The base or center of the device is fixed in its position with respect to the seabed.
A common problem with existing devices designed to harvest significant amounts of energy from the sea is their complexity and cost. They are predominantly large structures, with rigid components, placed in a harsh environment. There is little use of well-proven components. Most devices proposed are very demanding in terms of engineering design, deployment and maintenance.
Other known devices which are used in the marine environment, although not designed for the conversion of wave energy to usable power include devices designed to pump fluids from the sea-bed.
The term “wave motion” as used herein refers to both waves on a surface of a liquid and swell in a body of a liquid.
Ocean waves represent a significant energy resource. It is known to use a Wave Energy Converter to extract power from such waves. Known Wave Energy Converters tend to be expensive, and have limited prospects for survival in extreme conditions.
A variety of devices may be used to provide relatively small amounts of power for use in small devices intended for long life in inaccessible locations. For example, to perform long endurance military missions, small unattended sensors or robots need more electrical power to sense, communicate, or move than they can practically carry in a pre-charged power storage device. This means that they must be able to harvest energy from their environment during the mission to periodically re-charge their power sources.
The small size of the devices typically used in military systems makes it difficult to collect a useful amount of power since natural energy usually occurs as a “flux”, and the amount available for collection depends on the physical capture area.
The amount of energy or power available in waves is enormous and this power is generally recognized by the damage caused. Thus, waves are usually regarded as a hindrance rather than an asset. For example, at Wick Breakwater in Scotland a block of cemented stones weighing 1,350 tons was broken loose and moved bodily by waves. Several years later, a replacement pier weighing 2,600 tons was carried away.
In other instances, a concrete block weighing 20 tons was lifted vertically to a height of 12 feet and deposited on top of a pier 5 feet above the highwater mark; stones weighing up to 7,000 pounds have been thrown over a wall 20 feet high on the southern shore of the English Channel; and on the coast of Oregon, the roof of a lighthouse 91 feet above the water was damaged by a rock weighing 135 pounds.
Heretofore this enormous amount of power available in the world's oceans has been largely ignored. One reason for this lack of utilization of the available energy in the world's oceans is their very power. In other words, most devices which have been designed for capturing or converting the energy of waves to useful work have been destroyed or damaged by that very energy.
This is at least partly due to the irregularity of waves which can cause jerky or irregular motion of wave energy devices. Moreover, storms frequently occur during which time wave action can become violent, thus destroying installations erected for converting the energy of the waves to useful work.
Other prior art devices are not efficient in operation and convert only a very small portion of the available wave energy. For example, the actual propagation or movement of water particles in a lateral direction is only about one percent of the velocity of travel of waves. Thus, while devices floating on the surface of a body of water may be utilized to extract some of the energy of the waves themselves, these devices are not able to extract energy from the moving water itself.
Prior art devices range from elongate cylinders or like structures bobbing at the surface of the body of water for driving a propeller carried thereby, through so-called air turbines which comprise floating bodies at the surface with open bottom chambers into which waves are permitted to rise and fall for alternately compressing air in chambers to drive a turbine, up to complex bodies specifically configured to obtain rotational movement from the action of waves and moving water particles thereon to drive turbines. These last devices are commonly referred to as Salter's Duck.
All such prior art devices capture or convert only a small portion of the available power in waves and in many cases are not durable enough to withstand the forces encountered in the ocean's waters or are not cost efficient.
Another device provides a structure which floats at the surface of a body of water and is constructed to convert the rolling or orbital motion of water particles in the waves into a linear flow of water and to then accelerate the linear flow without using any mechanical means or process. The accelerated flow is then utilized, inter alia, to drive a water wheel, turbine or the like for extracting power from the moving body of water.
Another known a device for the conversion of the energy of the gravitational waves, i.e. sea and ocean wind-formed waves, or dead or ground sea swell in which a series of input parallel converters are connected by means of an input collector manifold with a turbogenerator which on its turn is connected by means of an output collector-manifold to a series of parallel output converters which let out the water in the low part of the wave. In such devices, the input and output converters have independent sources of gas under pressure. The device is maintained at a given level by means of a ballast system fitted to the converters and stabilizers.
The disadvantage of this device is the large number of input and output elements which makes this device very complicated. Furthermore, the flow should surmount the local resistances in its inflow in the input and outflow from the output collector or manifold as a result of which there occurs a decrease in the harnessed energy.
Another shortcoming of such a known device is that the independent sources of gas under pressure maintain the water in the input and output converters and this can vary over a wide range. When the level is very low, part of the gas can flow out of the converters and this results in a loss of part of the compressed air. Conversely, when we have a high level and a little volume of the gas cushion, the latter is inferior in its role as buffer and energy accumulator.
Furthermore, this device cannot be directed at a specified angle towards the wave front, and in this way an important reserve for increasing its smoothness of operation and improving its efficiency cannot be utilized.
In the past, research performed on ocean thermodynamics revealed that energy costs would surpass energy production for the then known ocean wave producing energy systems. In one known system, a floating tank is provided with an opening at the top of the tank which leads into a passage extending through the center of the tank where a propeller-like blade is mounted.
The action of the ocean waves causes water to flow into the opening at the top of the tank where such water falls onto the blade to rotate it and consequently produce electrical energy. One limitation of this system is that it can utilize only the amount of ocean water that flows into the tank opening to provide the dynamic force on the propeller blade, rather than being able to use the full force of the ocean wave.
Generating technologies for deriving electrical power from the ocean include tidal power, wave power, ocean thermal energy conversion, ocean currents, ocean winds and salinity gradients. Of these, the three most well-developed technologies are tidal power, wave power and ocean thermal energy conversion.
Tidal power requires large tidal differences which, in the U.S., occur only in Maine and Alaska. Ocean thermal energy conversion is limited to tropical regions, such as Hawaii, and to a portion of the Atlantic coast. Wave energy has a more general application, with potential along the California coast. The western coastline has the highest wave potential in the U.S.; in California, the greatest potential is along the northern coast.
Wave energy conversion takes advantage of the ocean waves caused primarily by interaction of winds with the ocean surface. Wave energy is an irregular and oscillating low-frequency energy source that must be converted to a 60-Hertz frequency before it can be added to the electric utility grid.
Although many wave energy devices have been invented, only a small proportion have been tested and evaluated. Furthermore, only a few have been tested at sea, in ocean waves, rather than in artificial wave tanks.
As of the mid-1990s, there were more than 12 generic types of wave energy systems. Some systems extract energy from surface waves. Others extract energy from pressure fluctuations below the water surface or from the full wave. Some systems are fixed in position and let waves pass by them, while others follow the waves and move with them. Some systems concentrate and focus waves, which increases their height and their potential for conversion to electrical energy.
A wave energy converter may be placed in the ocean in various possible situations and locations. It may be floating or submerged completely in the sea offshore or it may be located on the shore or on the sea bed in relatively shallow water. A converter on the sea bed may be completely submerged, it may extend above the sea surface, or it may be a converter system placed on an offshore platform. Apart from wave-powered navigation buoys, however, most of the prototypes have been placed at or near the shore.
The visual impact of a wave energy conversion facility depends on the type of device as well as its distance from shore. In general, a floating buoy system or an offshore platform placed many kilometers from land is not likely to have much visual impact (nor will a submerged system). Onshore facilities and offshore platforms in shallow water could, however, change the visual landscape from one of natural scenery to industrial.
The incidence of wave power at deep ocean sites is three to eight times the wave power at adjacent coastal sites. The cost, however, of electricity transmission from deep ocean sites is prohibitively high. Wave power densities in California's coastal waters are sufficient to produce between seven and 17 megawatts (MW) per mile of coastline.
As of 1995, 685 kilowatts (kW) of grid-connected wave generating capacity is operating worldwide. This capacity comes from eight demonstration plants ranging in size from 350 kW to 20 kW. None of these plants are located in California, although economic feasibility studies have been performed for a 30 MW wave converter to be located at Half Moon Bay. Additional smaller projects have been discussed at Fort Bragg, San Francisco and Avila Beach. There are currently no firm plans to deploy any of these projects.
As of the mid-1990s, wave energy conversion was not commercially available in the United States. The technology was in the early stages of development and was not expected to be available within the near future due to limited research and lack of federal funding. Research and development efforts are being sponsored by government agencies in Europe and Scandinavia.
Many research and development goals remain to be accomplished, including cost reduction, efficiency and reliability improvements, identification of suitable sites in California, interconnection with the utility grid, better understanding of the impacts of the technology on marine life and the shoreline. Also essential is a demonstration of the ability of the equipment to survive the salinity and pressure environments of the ocean as well as weather effects over the life of the facility.
Wave energy could easily replace fossil fuel energy in some areas along coasts, cutting down on greenhouse gas emissions and the atmospheric pollution caused by burning fossil fuels. The effects on the environment are generally minor, as the construction of a wave energy plant requires about the same area as a small harbor.
Ocean waves are created by the interaction of winds with the surface of the sea. They contain large amounts of energy stored in the velocity of the water particles and in the height of the mass of seawater in a wave front above the mean level of the sea (E.S.B.I. and E.T.S.U., 1997).
The amount of wind energy that can be transferred to the surface of the ocean to create the waves depends upon the wind speed, the distance over which it interacts-known as the fetch—and the duration for which it blows over the water. Due to the direction of the prevailing winds and the size of the Atlantic Ocean, North Western Europe including Britain and Ireland have one of the largest wave energy resources in the world (E.S.B.I. and E.T.S.U., 1997).
There are significant differences in seasonal levels of wave energy, but the long term output should be somewhat more predictable than with some other renewable resources (E.S.B.I. and E.T.S.U., 1997).
The principal ways of extracting energy from waves rely either separately or jointly on the surge, heave and pitch of the waves. The frequency of arrival of ocean waves is low (a few per minute). As electrical generators rotate at hundreds of revolutions per minute the conversion mechanism must produce a higher frequency rotation to generate electricity-which is a convenient energy transfer medium. This can be done by hydraulic pumps or pneumatic bags/chambers driving higher speed turbines and generators.
Such conversion mechanisms have been tested by the construction of many varieties of experimental laboratory models and by some small-scale devices in the open sea or large lochs.
Currently there are two types of shoreline device in operation. One is a shoreline or caisson breakwater oscillating water column driving a pneumatic Wells turbine in 10 to 25 metres of water. The second type is a tapered channel device that was developed in Norway. In this type, incoming waves travel up a tapering channel, overflow and fill a higher level reservoir. The enclosed water then drives a Kaplan hydroelectric turbine as it returns to the sea.
A floating offshore device known as the circular clam has been developed by Sea Energy Associates and Coventry University, Britain. The design comprises a floating twelve-sided hollow ring. Each of the twelve sides has a flexible membrane that inflates and deflates with the incoming wave action. The air passes via a central circular manifold through Wells turbines, which drive electrical generators.
In the long term, wave conversion devices positioned in deep water offshore may provide the most likely method of large scale energy recovery. Development of shoreline and near shore systems is, however, relatively further advanced although considerable proving work remains to be done if these are to be considered as reliable sources of electricity (E.S.B.I. and E.T.S.U., 1997).